This is a Continuation-In-Part application of International Patent Application serial No. PCT/JP01/02567 filed on Mar. 28, 2001, now pending.
1. Field of the Invention
The present invention relates to a synthetic silica glass member, to a photolithography apparatus and to a process for producing the photolithography apparatus. More specifically, it relates to a synthetic silica glass member for a photolithography apparatus used in an optical system with lenses, mirrors and the like for photolithography technology, in a specific wavelength range of no greater than 400 nm and preferably no greater than 300 nm, to a photolithography apparatus constructed using such optical members and to a process for producing the photolithography apparatus.
2. Related Background Art
Photolithography apparatuses known as xe2x80x9csteppersxe2x80x9d (projection exposure apparatuses) are conventionally used for photolithography techniques which accomplish exposure and transfer of intricate patterns of integrated circuits onto wafers of silicon or the like. The optical system of a stepper is constructed with an illumination optical system which uniformly irradiates light from a light source onto a reticle and a projection optical system which projects and transfers the integrated circuit pattern formed on the reticle onto a wafer at a ⅕ reduction, for example.
Recent higher integration of LSIs has required even shorter wavelengths for photolithography apparatus light sources, from g rays (436 nm) to i rays (365 nm) or even KrF (248.3 nm) or ArF (193.4 nm) excimer lasers. There has also been a corresponding demand for photolithography apparatuses allowing exposure with even finer minimum workability beam width. However, when the light source emits light with a wavelength range in the ultraviolet region, and particularly in the region below 250 nm, the light transmittance is too poor for practical use when employing lens materials used in illumination optical systems and projection optical systems designed for light with a longer wavelength region than i rays. Lens materials used in illumination optical systems and projection optical systems are therefore limited to silica glass and some crystal materials that exhibit high light transmittance.
When a silica glass member is used in the optical system of a photolithography apparatus, extremely high quality is required for exposure of the integrated circuit pattern with a large area at high resolution. For example, for a member with a relatively large diameter of about 200 mm, the refractive index distribution of the member must be on the order of no greater than 10xe2x88x926. It is also necessary to reduce the birefringence, i.e. minimize internal strain of the member, and this improves the uniformity of the refractive index distribution and is also important for an the resolution of the optical system. Consequently, there have been strict limitations on silica glass members made of silica glass alone that can be used for photolithography apparatuses employing ultraviolet light from an excimer laser stepper as the light source.
In addition, silica glass members used for photolithography apparatuses using ultraviolet light as the light source must have high transmittance (small loss factor) in addition to the aforementioned conditions. This is because a very large number of lenses are provided in the illumination optical system or projection optical system of a photolithography apparatus to compensate for aberration, and the optical loss of each individual lens can lead to reduced transmittance of the apparatus as a whole.
Silica glass includes fused silica glass obtained by melting natural crystal powder and synthetic silica glass obtained by chemical synthesis, but synthetic silica glass is characterized by being of high purity with low metal impurities, and having high transmittance for ultraviolet light of a wavelength of 250 nm or smaller. Homogeneous synthetic silica glass with a large aperture can also be produced due to the nature of the production process.
Even with synthetic silica glass, however, the action of a high power ultraviolet beam or excimer laser beam produces a 260 nm absorption band due to structural defects known as NBOHCs (Non-Bridging Oxygen Hole Centers, having the structure xe2x89xa1Sixe2x80x94O.) or a 215 nm absorption band due to structural defects known as Exe2x80x2 centers (having the structure xe2x89xa1Si., where xe2x89xa1 is not a triple bond but represents bonding to 3 oxygen atoms, and.represents an unpaired electron), and transmittance in the ultraviolet region is significantly reduced as a result. Because the center wavelengths of these absorption bands are close to the oscillation wavelength of the KrF excimer laser or ArF excimer laser used as the light source of the stepper, the transmittance of the optical system is considerably reduced by light absorption. A large 215 nm absorption band is also sometimes produced even with low total ultraviolet beam irradiation (=low irradiation energy or a short irradiation time). Since the absorption band is usually produced at the initial stage of irradiation of an excimer laser, the absorption is referred to as initial irradiation absorption. Thus, not all synthetic silica glass can necessarily be used for lens members of such apparatuses, and therefore synthetic silica glass with low initial irradiation absorption is necessary in order to satisfactorily guarantee practical performance for the apparatus.
To date it has been independently confirmed that the desired properties for an optical member used in a photolithography apparatus employing an ArF excimer laser or the like with a wavelength of 200 nm or less as the light source are a low loss factor before irradiation with an ArF excimer laser beam (wavelength: 193.4 nm) and low transmittance reduction even with prolonged irradiation with an ArF excimer laser (i.e. low prolonged transmittance variation); however, it has been difficult to obtain silica glass members satisfying both of these properties, and therefore no photolithography apparatus has existed with an ArF excimer laser employing such a member as the light source.
It is an object of the present invention, which has been accomplished in light of these problems, to provide a synthetic silica glass member which can increase the transmittance of a photolithography apparatus employing an ArF excimer laser as the light source and thereby exhibit adequate properties for practical use, as well as a high-resolution photolithography apparatus employing it and a process for its production.
The present inventors investigated the relationship between the properties of silica glass and the degree of the initial irradiation absorption in order to elucidate the cause of the initial irradiation absorption of silica glass for ultraviolet light. As a result, it was found that when silica glass is synthesized in a reducing atmosphere for doping of hydrogen in order to achieve enhanced durability, providing an atmosphere that is more strongly reducing than necessary results in silica glass containing a high degree of the structural defect xe2x89xa1Sixe2x80x94H (which is easily cleaved by ultraviolet irradiation with low energy density to become an Exe2x80x2 center) which is a cause of lower transmittance of the silica glass member, or in other words, that silica glass with a greater hydrogen molecule concentration tends to exhibit high initial irradiation absorption. It was found at the same time that silica glass containing virtually no hydrogen molecules exhibits reduced transmittance with prolonged irradiation with an ArF excimer laser.
Thus, the synthetic silica glass of the invention is a synthetic silica glass member used in a photolithography apparatus employing light in a wavelength range of 400 nm or less, wherein hydrogen molecules are present at a certain level and the xe2x89xa1Sixe2x80x94H concentration is lowered during synthesis by appropriately adjusting the hydrogen molecule concentration of the silica glass during synthesis in a reducing atmosphere, the member being characterized in that, upon 1xc3x97104 pulse irradiation with an excimer laser at an energy density from 0.1 xcexcJ/cm2xc2x7p to 200 mJ/cm2xc2x7p, the loss factor at 193.4 nm measured after irradiation (i.e. the initial irradiation absorption) is no greater than 0.0050 cmxe2x88x921, the hydrogen molecule concentration is from 1xc3x971016 molecules/cm3 to 2xc3x971018 molecules/cm3 and the loss factor before ultraviolet irradiation is no greater than 0.0020 cmxe2x88x921.
The photolithography apparatus of the invention is a photolithography apparatus comprising an exposure light source that emits light with a wavelength of 400 nm or less as the exposure light, a reticle with a pattern image formed therein, an illumination optical system that irradiates light outputted from the exposure light source onto the reticle, a projection optical system that projects the pattern image outputted from the reticle onto a photosensitive plate and an alignment system that positions the reticle and the photosensitive plate, wherein at least some of the synthetic silica glass members composing the illumination optical system, the synthetic silica glass members composing the projection optical system and the reticle (for example, the collimator lens, flyeye lens, condenser lens, projection lens, etc. in the described embodiment) consist of synthetic silica glass members characterized as described above.
By constructing all or some of the optical members of a photolithography apparatus (for example, the stepper 1 in the described embodiment) using synthetic silica glass members according to the invention, it is possible to ensure high transmittance and adequate properties for practical use even when the light source is an ArF excimer laser. The synthetic silica glass member has a hydrogen molecule concentration of 1xc3x971016 molecules/cm3 to 2xc3x971018 molecules/cm3 and a loss factor of no greater than 0.0020 cmxe2x88x921 before ultraviolet irradiation.
A hydrogen molecule concentration of less than 1xc3x971016 molecules/cm3 results in greater reduction in transmittance with prolonged irradiation with an ArF excimer laser. On the other hand, synthesis under conditions with a hydrogen molecule concentration of greater than 2xc3x971018 molecules/cm3 tends to result in a higher xe2x89xa1Sixe2x80x94H concentration as well and greater initial irradiation absorption, and a non-uniform hydrogen molecule concentration in the silica glass.
A photolithography apparatus constructed using a synthetic silica glass member with a loss factor of greater than 0.0020 cmxe2x88x921 before ultraviolet irradiation has low transmittance (throughput) for the optical system as a whole, and lower resolution is exhibited due to heating of the member upon irradiation with the laser or changes in the surface shape. In the case of a synthetic silica glass member with a loss factor (initial irradiation absorption) exceeding 0.0050 cmxe2x88x921 at 193.4 nm measured after irradiation with 1xc3x97104 pulse irradiation at an energy density of 0.1 xcexcJ/cm2xc2x7p to 200 mJ/cm2xc2x7p, a large variation in transmittance is exhibited when the laser is switched on or off, and therefore the photolithography apparatus employing it has a large light exposure variation.
The process for producing the photolithography apparatus of the invention comprises an ingot synthesizing step in which a synthetic silica glass ingot with a hydrogen molecule concentration of from 1xc3x971016 molecules/cm3 to 2xc3x971018 molecules/cm3 is synthesized; an ingot cutting step in which the synthetic silica glass ingot is cut to obtain a synthetic silica glass member of the prescribed shape and size; a loss factor measuring step for measurement of the loss factor of the synthetic silica glass member before ultraviolet irradiation and the loss factor at 193.4 nm after irradiation with an ArF excimer laser with 1xc3x97104 pulse irradiation at an energy density from 0.1 xcexcJ/cm2xc2x7p to 200 mJ/cm2xc2x7p, to obtain a synthetic silica glass member with a loss factor of no greater than 0.0020 cmxe2x88x921 before ultraviolet irradiation and a loss factor of no greater than 0.0050 cmxe2x88x921 at 193.4 nm; and an optical system constructing step in which the synthetic silica glass member obtained in the loss factor measuring step is used to construct at least some of the synthetic silica glass members composing the illumination optical system, the synthetic silica glass members composing the projection optical system and the reticle.
According to this production process, it is possible to obtain a high-resolution photolithography apparatus employing a synthetic silica glass member with a loss factor of no greater than 0.0020 cmxe2x88x921 before ultraviolet irradiation and a loss factor of no greater than 0.0050 cmxe2x88x921 at 193.4 nm.
According to the invention, the distribution range (variation width) is preferably small for the hydrogen molecule concentration value in the radial direction at any given point of the synthetic silica glass member. The radial direction is the direction along any desired plane in the member perpendicular to the direction of the incident irradiated beam. Specifically, for the hydrogen molecule concentration at the center of the member (geometrical center of gravity), the variation width V of the hydrogen molecule concentration at any desired point on the plane including this center preferably satisfies the condition represented by xe2x88x9250%xe2x89xa6Vxe2x89xa6+50%, and more preferably satisfies the condition represented by xe2x88x9220%xe2x89xa6Vxe2x89xa6+20%. Thus, with a small distribution range (variation width) for the hydrogen molecule concentration in the radial direction of the member, it is possible to achieve a stable exposure dose over a prolonged period without irregularity in the degree of reduction in transmittance at any point in the member in the radial direction of the member, even when the ArF excimer laser is irradiated thereon for a prolonged period.
As explained above, when the hydrogen molecule concentration exceeds 2xc3x971018 molecules/cm3 the variation width of the hydrogen molecule concentration tends to be larger. In order to reduce the variation of the hydrogen molecule concentration, it is preferred to adjust the oxygen gas/hydrogen gas flow ratio to 0.25-0.40 for the total combustion gas (oxygen-containing gas and hydrogen-containing gas) ejected from the burner during the silica glass ingot synthesis step. If the flow ratio is within this range it will be possible to include the necessary number of hydrogen molecules in the silica glass to prevent reduced transmittance during prolonged irradiation of light, while also minimizing the degree of xe2x89xa1Sixe2x80x94H structure which is a cause of initial irradiation absorption, and to thereby reduce the variation width for the hydrogen molecule concentration; in addition, since the various conditions for the ingot synthesis step such as the synthesis burner shape, the raw material flow, the oscillating pattern of the target holding the ingot, etc., are also factors determining the hydrogen molecule concentration in the ingot and its variation width, the xe2x89xa1Sixe2x80x94H concentration, etc., appropriate adjustment of these conditions is preferred. For example, if the raw material flow is too high the center temperature of the flame falls, tending to result in easier hydrogen concentration distribution.